Moments (Edexcel GCSE Physics)

A student investigates moments of forces.

Figure 14 shows the apparatus used.

Figure 14

The pivot is under the centre of the rod.

A magnet is fixed to one end of the rod.

A piece of modelling clay is fixed to the other end of the rod.

The system is in equilibrium.

State the relationship between the moment of the weight of the magnet and the moment of the weight of the piece of modelling clay about the pivot.

The student fixes a coil to the bench under the magnet as shown in Figure 15.

Figure 15

The coil of wire is connected to a d.c. power supply so that there is a current in the coil.

To bring the system back into equilibrium, the student hangs a 0.050 N weight on the rod, 8.4 cm away from the pivot, as shown in Figure 15.

Calculate the size of the force between the magnet and the coil.

force = .............................................................. N

Describe how the student could develop the investigation to determine if the size of the force between the magnet and the coil is directly proportional to the size of the current in the coil.

The student reverses the direction of the current in the coil.

Describe how the student can bring the system back into equilibrium without making any changes to the magnet.

Figure 19 shows four forces, P, Q, R and S, acting on a rod. The rod can rotate around an axle.

Figure 19

Which force will make the rod rotate about the axle?

Figure 20 shows a person trying to lift a large rock using a metal bar.

Figure 20

The rock weighs 1800N.

The person can only produce a downwards force of 600N.

The person cannot lift the rock.

i) Explain, using calculations, why the person cannot lift the rock.

[3]

ii) Explain one change to the arrangement that will make it possible for this person to lift the rock.

[2]

Figure 21 shows a bicycle.

Figure 21

i) The rider uses the pedals to make the large gear wheel turn.

The large gear wheel moves the chain. The chain turns the small gear wheel.

The large gear wheel has 48 teeth. The small gear wheel has 12 teeth.

The large gear wheel turns 2 times each second.

Calculate the number of times that the small gear wheel turns each second.

............................................... turns each second[2]

ii) Oil is applied to the wheel of a bicycle at the point shown in Figure 22.

Figure 22

Explain how the oil improves the efficiency of the bicycle.

[3]

Physics Only

i) Figure 9 shows two gears.

Gear Q moves clockwise as shown by the arrow.

Figure 9

Which of these describes how gear P moves?

[1]

ii) Figure 10 shows the number of teeth on the gears.

Gear P has 24 teeth, gear Q has 16 teeth.

Figure 10

Which of these gives the ratio of the number of teeth on gear P to the number of teeth on gear Q?

[1]

Figure 11 shows a lever used to lift a heavy load.

Figure 11

i) The weight of the load is 650N.

The centre of the load is 0.75m from the pivot.

Calculate the moment of the load about the pivot.

State the unit.

Use the equation      

moment = force × distance from the pivot

moment = ................... unit..............................[3]

ii) State the principle of moments.

[1]

iii) An effort of 160 N is applied to the end of the lever to balance the load in Figure 11.

Calculate the distance between the effort and the pivot.

distance = ............................................... m[3]

i) Figure 6 shows two gears.

Figure 6

Gear R and gear S can rotate.

Gear R has 20 teeth.

Gear S has 60 teeth.

Gear S rotates through 2 complete revolutions.

Calculate how many complete revolutions gear R rotates by.

gear R has rotated through ............................. revolutions[2]

ii) Figure 7 shows two gears, S and T.

Figure 7

Gear S can rotate on a fixed axle.

Gear T can move up and down.

Gear S has 60 teeth.

The distance between each of the teeth on gear S and on gear T is 2 mm.

Gear S moves through one complete revolution in the direction shown.

Which of these describes the motion of gear T?

[1]

i) Figure 8 shows a force of 70 N turning a lever about point P.

Figure 8

Calculate the moment of the 70 N force about point P.

State the unit.

Use the equation

moment = force × distance normal to the direction of the force

moment = ..................... unit ...............[3]

ii) Figure 9 shows a worker using a wheelbarrow to move some sand.

Figure 9

The load is equal to the total weight of the sand and the wheelbarrow. The effort is the force that the worker applies to the wheelbarrow handles.

The worker applies just enough effort to lift the load.

Explain why the effort is smaller than the load.

[2]

iii) Some sand falls down and sticks between the wheel and the axle.

State why it might be harder to push the wheelbarrow along when there is some sand between the wheel and the axle.

[1]

Figure 10 shows an arrangement of gears.

Each gear turns around a fixed axle.

Figure 10

Gear P is turned through one complete revolution per second.

i) Which of the following describes the motion of gear R?

[1]  

ii) Figure 11 shows the same arrangement with gear R replaced by a rack.

Figure 11

The rack can move up or down when the gears turn.

The teeth on the rack are 2 mm apart.

Calculate how far the rack moves when P turns through to one complete revolution.

distance = ............................................... mm[2]

Figure 12 shows three toy animals hanging from a rod.

The rod hangs from the ceiling by a string tied to the centre of the rod.

The system is in equilibrium.

Figure 12

Use the principle of moments to calculate the mass of the toy elephant.

Figure 13 shows a diagram of a device for lifting heavy loads.

Figure 13

The metal tube is filled with oil.

The piston Y is pushed down with a force K.

This produces a force L on piston Z.

The pressure exerted on the oil by piston Y is the same as the pressure exerted by the oil on piston Z.

Explain the difference between the size of force K and the size of force L.

Higher Tier Only

Figure 1 shows some forces acting on a seesaw.The forces shown have the same magnitude but act in different directions.

Figure 1

In which of these could the forces acting on a seesaw be in equilibrium?

Higher Tier Only

Figure 2 shows an open door.

Figure 2

Explain why it is easier to close the door by pushing at point P rather than pushing at point Q.

Higher Tier Only

Figure 3 is a diagram of the forces acting on a swing. The swing is not moving.

Figure 3

The seat of the swing, AB, is 0.80 m long. A person of weight 450 N sits on the seat. The person’s weight acts at a distance of 0.50 m from A as shown in Figure 3.

Ignore the weight of the seat.

The upward forces exerted by the ropes on the seat are F₁ and F₂.

Calculate the force F₂ by taking moments about A.

force F₂ = ...............................N